Field of the Invention
The present invention relates to a turbomachine casing, more particularly a turbomachine such as a turbojet or a turboprop engine in a plane.
Description of the Related Art
A fan casing typically comprises a substantially cylindrical wall which extends about the turbomachine fan blades, and the internal surface of which is covered with acoustic insulation panels. Such panels generally comprise an annular honeycomb structure the internal and external surfaces of which are each covered with a skin which can be multi-perforated to improve the acoustic treatment. They are intended to absorb the sound waves generated by the fan of the turbomachine.
The applicant's previous applications EP-A1-2318679, EP-A1-2,088,290 and FR-A12965859 describe acoustic insulation panels of this type.
An acoustic insulation panel in a turbomachine is generally sectorized, i.e. formed of several panel sectors circumferentially arranged end-to-end and secured to the casing wall by gluing or fixing screws which extend radially with respect to the longitudinal axis of the turbomachine. Fixing the sectors by screwing has disadvantages in that it requires using many fixing screws which, on the one hand, increase the weight of the turbomachine and are liable to damage the fan blades in case of loss or breakage. On the other hand, more or less wide interface areas between the sectors induce alternating smooth and treated areas in the vicinity of the fan (and thus the creation of acoustic impedance discontinuities), which cause an increase in the noise levels in the turbomachine for some operating conditions. Besides, mounting the numerous fixing screws on the panels requires the provision therein of densified zones that affect the acoustical effectiveness.
In order to limit the number of fixing screws, the above-mentioned patent application FR-A1 2,935,017 provides to extend the fan casing and to fix an annular one-piece acoustic insulation panel on the casing wall and the air inlet duct. In practice, the maintenance of such an acoustic panel is difficult to achieve since the turbomachine has to be immobilized to substitute a new panel for the damaged panel.
Furthermore, a casing wall may be deformed and have an internal surface which is not perfectly cylindrical. This is more particularly the case in a casing wall made of a composite material which, because of the method used for the manufacturing thereof, may have relatively important manufacturing tolerances. In the currently available technique, the acoustic panels which are fixed on a casing wall of this type do not make it possible to compensate the above-mentioned tolerances.
This is more particularly the case for acoustic insulation panels which are glued to the casing wall by means of a hot-setting adhesive. Implementing such gluing is long (several hours are requested in a large-sized autoclave) and, if the panel is damaged, the engine must be dismounted prior to replacing the panel.